The influence of the localization of the placenta and some technical problems associated with the performance of amniocentesis (AC) on the incidence of spontaneous abortion (SA) after AC was evaluated in a prospective study comprising all women (2276) referred for AC at the University Hospital in Odense during a 7-year period. Women with predisposing factors for SA were excluded from this analysis, which comprised 1545 women. Of these, 1289 women had an AC and 256 were judged not to need an AC after ultrasonographic examination. The localization of the placenta per se had no influence on the incidence of SA. However, if the placenta was covering the whole anterior wall so that perforation of the placenta could not be avoided, or if more than one insertion was necessary, or there was macroscopic blood contamination in the amniotic fluid, the risk of SA was increased by a factor 4 to 5. The influence of previous obstetric or gynecologic complications on the incidence of SA was also examined. In this analysis the data from women with first trimester hemorrhage in the present pregnancy were included and the study population therefore consisted of 1594 women. Of these, 1318 had an AC, and 276 had ultrasound scanning only. Patients with one or more previous pregnancies with fetal loss had a significantly greater risk of SA after AC than patients with no previous pregnancies or successfully completed pregnancies. Two subgroups with special problems, namely, women with previous complaint of infertility of at least 2 years' duration and women with first trimester bleeding, also had an increased risk of SA.(ABSTRACT TRUNCATED AT 250 WORDS)